MONDAY, Oct. 30, 2017 (HealthDay News) -- Acetaminophen is considered the go-to pain medication during pregnancy. But a new study adds to evidence linking the drug to an increased risk of behavioral issues in kids.

The link was, however, confined to longer-term use -- particularly a month or longer.

When moms used acetaminophen for 29 days or more during pregnancy, their kids were twice as likely to be diagnosed with ADHD, versus women who did not use the drug.

On the other hand, when expectant moms used the drug for a week or less, their kids showed a slightly decreased risk of ADHD.

Acetaminophen is best known by the brand name Tylenol, but it's an active ingredient in many pain relievers.

The new study, led by researcher Eivind Ystrom from the Norwegian Institute of Public Health in Oslo, is not the first to suggest a connection between prenatal acetaminophen and ADHD.

But experts said it's still hard to definitively pin the blame on the medication.

"That's the dilemma," said Christina Chambers, co-director of the Center for Better Beginnings at the University of California, San Diego.

Around half of pregnant women use acetaminophen at some point, so it's important to understand any risks, according to Chambers, who was not involved in the study.

But with a study like this, she explained, it's difficult to know whether factors other than acetaminophen are to blame -- including the underlying conditions the women had.

According to the study researchers, longer-term use was tied to ADHD whether women used it for pain, fevers or infections.

But if a woman was using the medications for weeks to treat a fever or infection, that indicates she was quite ill, Chambers pointed out.

And if she took the drug for chronic pain, Chambers said, that raises the question of what effects the pain condition could have on her pregnancy.

For now, Chambers stressed that pregnant women should not be scared off from using acetaminophen for a fever -- since an untreated fever could carry risks.

"The last thing we'd want, heading into flu season, is for women not to use acetaminophen to get a fever down," she said.

"This study," Chambers added, "suggests that if there is a causal association between acetaminophen and ADHD, it's with more-chronic use."

Overall, more than 2,200 children in the study were diagnosed with ADHD -- or about 2 percent of the whole group. The risk was just over twofold higher among kids whose mothers had used acetaminophen for 29 days or more during pregnancy.

The drug might, for instance, interfere with maternal hormones that are important for fetal brain development.

But even if long-term acetaminophen does influence ADHD development, Chambers said, this study suggests it's a "modest" effect.

"The risk to any one woman would be small," she said.

That said, Chambers pointed to a bigger-picture issue: Very few drugs have actually been studied in pregnant women, and fairly little is known about the safety of using any medication prenatally.

The study was published online Oct. 30 in the journal Pediatrics.

Dr. Mark Wolraich, a professor of pediatrics at the University of Oklahoma Health Sciences Center, wrote an editorial accompanying the study.

He agreed that the study points only to an association between acetaminophen and ADHD, which might reflect a "third factor" at play, such as the underlying condition that caused the women to take the drug.

Plus, Wolraich explained, many factors might affect the development of ADHD. The evidence is "much stronger" for a familial influence, since the disorder tends to run in families, he noted.

Still, Wolraich said, pregnant women may want to be "overly cautious" about using acetaminophen for any extended time. He suggested that women talk their doctor before using any medications.